The present invention relates to a vehicle behavior control system, such as a yaw rate control system, for controlling behavior of a vehicle, and more specifically to a vehicle behavior control system devised to provide adequate control performance just after a start of the system.
A Japanese Patent Provisional (Kokai) Publication No. 6(1994)-219300 discloses a conventional vehicle behavior control system for controlling the vehicle behavior by varying a rear wheel steer angle with an electric motor. From turn-on of the ignition switch until detection of the neutral steering position, this control system is unable to determine the steering wheel angle relative to the neutral position, and the behavior control based on the steering angle remains inaccurate, incurring the possibility of unwanted rear wheel steering operation. Therefore, the behavior control by the rear wheel steering is not initiated until the vehicle is brought to a neutral state in which the neutral position of the steering wheel can be detected, the rear wheel steering position is around the neutral position, and the target value of the rear wheel steer angle is in a neutral region.
A Japanese Patent Provisional (Kokai) Publication No. 5(1993)-310141 shows another behavior control system. When the neutral signal representing the neutral position of the steering wheel is not available, this system controls the rear wheel steer angle by determining a target rear wheel steer angle from a vehicle speed and a sensed yaw rate without using the steering wheel angle as input information.
When the vehicle is started from the resting state with the steering wheel turned away from the neutral position, the former control system must defer a start of the behavior control until the steering wheel is returned to the neutral position. In the latter system using the sensed yaw rate to determine the target motion variable, calculation of the target behavior causes a response delay, and tends to incur hunting in the behavior control.
Some problems can be solved by a high accuracy absolute steering angle sensor. However, such a sensor is costly.